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Beaverton, Oregon is a suburb within the Greater Portland metropolitan area, located eight miles west of the city center. All of the Greater Portland area is within the Coast Range Province of the Rocky Mountains and just above the confluence of the Willamette and Columbia Rivers. Beaverton is in the valley of the Tualatin River, which flows east from the Coast Range to join the Willamette River as it continues north along the west face of the Cascade Range toward Portland.

Oregon Coast Range

The Coast Range is part of the Cascadia Subduction Zone, where the North American continent is moving westward over the Juan de Fuca plate in the Pacific Ocean. The Coast Range rose during the middle Eocene, 50 million years ago, originally as a seabed volcanic arc, and was uplifted by rifting, or movement along a fault line. All of the Portland area is in a fore-arc basin, or lowland developed over a former seabed. The Tualatin sub-basin at Beaverton is the northernmost of a regional sequence of small basins separated by folded uplands capped by hard, weather-resistant lava flows of the Columbia River Basalt Group.

Columbia River Basalt Group

The Columbia River Basalt Group (CRBG) is an accumulation of volcanic flows that date from the Miocene, which began 23 million years ago. The CRBG is found throughout northern Oregon, southern Washington and western parts of the Idaho panhandle. In the Tualatin Basin near Beaverton, as much as 1,000 feet of Miocene CRBG flowed over the underlying sediments during volcanic floods that varied in thickness from 10 to 100 feet. Remains of ancient soil development are found between the flows.

Cooper and Bull Mountains

The southeastern half of the Tualatin Basin is separated into sub-basins by the anticlines, or upwardly bowed folds, of Cooper Mountain and Bull Mountain. The mountains were folded upward by tectonic plate movement in the subduction zone. Boreholes made at the top of Cooper Mountain measured its CRBG cap at 958 feet thick, indicating that the mountaintop was at a low elevation when lava flows ponded and cooled there. Cooper Mountain is the site of a 230-acre nature preserve.

Beaverton Fault Zone

Beaverton is built on recent sediments washed into the basin from nearby mountains, and such unconsolidated sediments behave like liquid rather than solid rock during seismic events, thus amplifying earthquake danger and damage to buildings. The eastern point of the Beaverton Fault Zone, an area of active earthquake potential, is below Beaverton city center and extends 8.7 miles west, following the northern base of Cooper Mountain. Vertical displacement along the deeply buried fault line has been established by measuring the relative position of the CRBG and other formations, using boreholes and water well cores.

About the Author

Sara Kirchheimer holds a Bachelor of Science in physical geography from Arizona State University and is currently retired from the transportation and travel industry in northern Europe and the western United States. In addition to commercial writing, she has contributed art exhibit reviews to Phoenix Arts and hurricane update articles to New Orleans Indymedia.

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